Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charged-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment. In particular, power MOSFETs are commonly used in electronic circuits, such as communication systems and power supplies, as electric switches to enable and disable the conduction of relatively large currents in DC to DC converter applications.
The power MOSFET includes a large number of MOSFET cells or individual transistors that are connected in parallel and distributed across a surface of a semiconductor die. To maximize power conversion efficiency, the power MOSFETs must exhibit low conduction and switching losses. Conduction losses are proportional to the drain to source resistance in the operational state (RDSON) of the transistor. Switching losses are proportional to the switching frequency and internal parasitic capacitance, most significantly gate to drain capacitance (Cgd). The trench power MOSFET is widely used due to its characteristic low RDSON. However, trench power MOSFETS commonly exhibit high Cgd. The trench MOSFET structure can be modified to improve Cgd, but at the expense of significantly increased manufacturing complexity.
A lateral MOSFET has a very small gate drain overlap resulting in a significantly lower Cgd than the trench MOSFET. The low Cgd makes the lateral MOSFET well suited for high frequency switching applications. A weakness of the lateral MOSFET structure is a higher RDSON compared to the trench MOSFET. The cell pitch of the lateral MOSFET includes an extended drift region to support the required blocking voltage of the device. The extended drift region requires a larger cell pitch and therefore higher RDSON. There remains a need for power MOSFET structure with improved device performance, i.e., low Cgd and low RDSON, and efficient manufacturability.